Could we push more than one buffer in pad chain function?

[Jake Zhang]

Greetings,

I am implementing a fairly complicate custom plugin and this plugin sometimes will hold on some input buffers by pushing them into a queue if some timestamp condition does not meet yet, and sometimes it might push more than one buffer to srcpads if certain condition is met. Do you guys see any issue of doing this? I just want to make sure I will not have a big issue with this unconventional chain function. 

I have went through the gstpad.c source code, and if my understanding is correctly the pad chain function will be called once every time each time there is a input buffer. And I do not see any issue with my above implementation. 

Please correct me if I am wrong. Any input is appropriated.

Thanks

Jake

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[Nirbheek Chauhan]

On Tue, Sep 6, 2016 at 5:21 AM, Jake Zhang <[hidden email]> wrote:

> I am implementing a fairly complicate custom plugin and this plugin
> sometimes will hold on some input buffers by pushing them into a queue if
> some timestamp condition does not meet yet, and sometimes it might push more
> than one buffer to srcpads if certain condition is met. Do you guys see any
> issue of doing this? I just want to make sure I will not have a big issue
> with this unconventional chain function.
>
> I have went through the gstpad.c source code, and if my understanding is
> correctly the pad chain function will be called once every time each time
> there is a input buffer. And I do not see any issue with my above
> implementation.
>
> Please correct me if I am wrong. Any input is appropriated.
>

Yes, it is a very common use-case to not always push a single buffer
when chaining a buffer. All you have to do is make sure that you
return GST_FLOW_OK when everything is fine. You can either push
nothing, a single buffer, or multiple buffers, or a buffer list.

There's roughly three things you can do here, depending on what your
data looks like.

If your data is meant to be concatenated/merged, then you can merge
the contents of buffers and push them as a single buffer as needed.
One way to do this is with GstAdapter[1]. This is commonly done in,
for instance, payloaders and depayloaders.

1. https://developer.gnome.org/gstreamer-libs/stable/GstAdapter.html

If you want to keep GstBuffers as they are, you can maintain a GList
or GPtrArray of them and push multiple ones individually
(gst_pad_push) in a single chain function invocation (be careful to do
error checking for each push!). The third option is to both store and
push them as a single GstBufferList[2] (gst_pad_push_list).

2. https://developer.gnome.org/gstreamer/stable/gstreamer-GstBufferList.html

The first and third options have the least overhead, if that matters
for your use-case.

Cheers,
Nirbheek

--
~Nirbheek Chauhan
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[Jake Zhang]

Thanks Nirbheek. Your answer is exactly what I am looking for. Really appreciate it. 

Jake

On Mon, Sep 5, 2016 at 11:25 PM, Nirbheek Chauhan <[hidden email]> wrote:

On Tue, Sep 6, 2016 at 5:21 AM, Jake Zhang <[hidden email]> wrote:
> I am implementing a fairly complicate custom plugin and this plugin
> sometimes will hold on some input buffers by pushing them into a queue if
> some timestamp condition does not meet yet, and sometimes it might push more
> than one buffer to srcpads if certain condition is met. Do you guys see any
> issue of doing this? I just want to make sure I will not have a big issue
> with this unconventional chain function.
>
> I have went through the gstpad.c source code, and if my understanding is
> correctly the pad chain function will be called once every time each time
> there is a input buffer. And I do not see any issue with my above
> implementation.
>
> Please correct me if I am wrong. Any input is appropriated.
>

Yes, it is a very common use-case to not always push a single buffer
when chaining a buffer. All you have to do is make sure that you
return GST_FLOW_OK when everything is fine. You can either push
nothing, a single buffer, or multiple buffers, or a buffer list.

There's roughly three things you can do here, depending on what your
data looks like.

If your data is meant to be concatenated/merged, then you can merge
the contents of buffers and push them as a single buffer as needed.
One way to do this is with GstAdapter[1]. This is commonly done in,
for instance, payloaders and depayloaders.

1. https://developer.gnome.org/gstreamer-libs/stable/GstAdapter.html

If you want to keep GstBuffers as they are, you can maintain a GList
or GPtrArray of them and push multiple ones individually
(gst_pad_push) in a single chain function invocation (be careful to do
error checking for each push!). The third option is to both store and
push them as a single GstBufferList[2] (gst_pad_push_list).

2. https://developer.gnome.org/gstreamer/stable/gstreamer-GstBufferList.html

The first and third options have the least overhead, if that matters
for your use-case.

Cheers,
Nirbheek

--
~Nirbheek Chauhan
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[hidden email]
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